Does anyone know if there is any way or retrofit to gain access to the two audio tracks which can only be used with the internal mic? I frequently need more than two audio channel inputs and it would be nice to be able to input to those additional channels

No, that can't be done.

Keith, this video might be of interest to you.

XDCAM ESSENTIALS Episode 6: Recording Four Channels of Audio on Vimeo

Thanks, Doug. I've been using channel's 3 and 4 as a back up with auto limiter for channel's 1 and 2. It would be great if there was a way to input a 3rd and 4th input source into the last two track though. That way I could capture four wireless mics at once or two mics and two channels off a mixing board.